China News Service, February 10 (Guo Chaokai, Zheng Yingying) As the Tianwen-1 Mars Rover successfully "brakes" and enters orbit around Mars, China's first autonomous Mars exploration mission "round, fall, and patrol" among the three major targets The surrounding goal was successfully achieved.
From the overall point of view, the "Mars Journey" has several highlights:
Effect Picture Courtesy of the Eighth Academy of China Aerospace Science and Technology Corporation
"Just walk away" travel? No it's not that simple
This time, "Tianwen-1" successfully braked into an orbit around Mars, which is of great significance, because "Mars travel" is not something you can go.
To start, an escape velocity of up to 70km/s (kilometers per second) is required, while the current maximum escape velocity provided by a launch vehicle is only about 15km/s.
In order to save energy as much as possible, we have to wait for the Earth and Mars to revolve to a specific relative position to launch the Mars probe.
According to the calculation of the revolution period, the rendezvous period between the Earth and Mars is about 778 days (that is, about 26 months), which means that the Earth and Mars will only have a "launch window" every 26 months (each window lasts about 20 days) ), once you miss a "launch window", you can only wait for the next time.
Therefore, in the Mars exploration missions planned for 2020, in addition to China’s Tianwen-1 Mars rover, there are also the U.S. Mars rover Perseverance, and the UAE, which has just announced its successful entry into Mars orbit on February 9, local time. The "Hope" Mars rover.
"Interstellar highway" that cannot be separated from "pork chops"
According to the launch window in 2020, the China Aerospace Science and Technology Group's development team completed the design of the transfer orbit from the Earth to Mars, and customized an "interplanetary highway" to Mars for the probe.
Speaking of "ground-fire transfer track design", one cannot fail to mention "Pork-Chop".
When designing interplanetary exploration orbits, designers will draw contour maps of the required launch energy for different launch dates and arrival dates. This contour map resembles two pork chops, named "Pork-Chop" This makes it known in the industry.
In order to allow the Mars rover to bring as many instruments and equipment as possible and see enough of Mars at a time, orbit designers need to find the most energy-efficient ground fire transfer orbit.
To this end, the designers first simplified the design of the complex earth-fire transfer orbit into a two-body gravity model of "Earth + Mars". By solving this model, the energy required for each pair of launch and arrival dates in the window was obtained. In order to draw the emission energy contour "pork chops".
Then, using the date selected on the "pork chop" chart as the initial value, using mathematical methods such as differential correction, iterative calculations in a certain precision Mars exploration orbital dynamics model, to obtain the "ground-fire transfer" for flight missions Orbital design" results.
Highly difficult "diving" in one go
After successfully "braking" into the orbit around Mars, the orbiter will implement another key action of the "Mars Journey" in a few months-the separation of the machine and the machine.
In about 3 hours, the orbiter needs to change its orbit to a dangerous Martian orbit, and establish and maintain the attitude required for the lander to enter the Martian atmosphere (the attitude error is less than 0.01 degrees).
At the scheduled separation time, the orbiter and the lander must complete the separation. After a safe distance flying process, the orbiter needs to quickly complete thrust acceleration in order to return to a safe orbit around Mars.
To put it vividly, the orbiter is like an excellent diving athlete who has to complete a difficult toss in the air and a precise time into the water.
Moreover, the series of posture maneuvering "toss" and "water entry" actions of the orbiter must be completed in one go.
"Into the water" too early, it is difficult to ensure the accuracy of the lander's entry. "Into the water" too late, it will cause the risk of the orbiter hitting Mars. This set of "super-difficult actions" also needs to be completed autonomously without the support of real-time monitoring and control on the ground.
It is reported that the development team carefully considered the potential risks of the separation process, and has designed an autonomous handling plan in the event of failure of various factors such as attitude measurement, speed calculation, and thrust output.
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